Hey GuysI have successfully used my Arduino and a couple of 4N35 opto's to press buttons a remote control for my garage doors, alarm etc. Unfortunately, the remote control's wireless signal for the gate does not reach all the way to the gate, so I had an intercom system installed. On the "phone" inside the house is a Aiphone VC-K which has a gate button.I checked the schematics of the phone, pressing the button simply shorts the "gate"-wire and ground together (pin 3 and 4 on the phone connector).I then connected a piece of wire to those terminals and shorting them together indeed opens the gate. Then I connected the wires to the detector pins of the 4N35, but it won't open the gate. I tried swapping the polarity of the wires as well, but still nothing.

When you are listening over the intercom system and press the gate button, there is definite click over the speaker, but when the button is "shorted" by the opto, the click isn't realy as loud. I still have to measure the current required to short the connection, but I suspect the 4N35 can't handle the current. The datasheet gives a maximum of 150mW of power, so if the gate switch is at 12V thats a maximum of about 12mA that may flow through the opto right?

If it is indeed the opto that can't handle the current, would it be possible to use n-type MOSFET (I have lots of IRF530) to short the circuit?

I'm just not sure how to put it in the circuit, since I only have ground and the pin that needs to be shorted to ground of the intercom circuit available.

Yes, you should determine what the open switch voltage level is and if AC or DC and how much current flows through the switch when pressed. Only then can you come up with a proper interface method. Or if you can't make such measurements, you could use a arduino controlled relay and wire the relay contacts across the existing switch contacts.

Measurements were taken using a multimeter and scope between the intercom gate and ground wires.Voltage open circuit: 13.4V DC (The scope shows a average 13.4V with a fluctation of 200mV over 10 seconds)Current to short gate wire to ground wire: 34mA (measured using multimeter inline with pushbutton)

Please see the attached breadboard diagram of how the measurements were taken and how to Arduino is connected via the 4N35. The resistor value is incorrect in the diagram, it is really a 380 ohm resistor to give the 4N35 1.2V 10mA.

Is my only option here a relay? Is there any other way I keep the solution as solid state as possible without physically connecting the intercom system to the Arduino circuit?